Questions and Answers

BMW 320i Fuel System Problems Q. I have a 1981 BMW 320i. Changed both fuel pumps, relay, fuse and hoses. Still not pumping fuel through pumps. WHAT DID I MISS? And where do I go from here? I have ½ tank of fuel and spark to the plugs. The books I have do not show other relays or what and where to look, just to test them. I need some help.

A. What you are going to have to do is check the wiring. I would start at the fuel pump and work forward.

Check for power at the green/black wire on both fuel pumps while cranking the engine. If you have no power there, move forward to the fuel pump relay.

With they key in the ON position, you should have power to terminal 15, green/violet wire. If not, there is an open between fuse three and the fuel pump relay. Then check for power at the green/black at terminal 87 of the fuel pump relay. If you have power there, you have an open between the fuel pump relay and the fuel pumps.

Now check for power at terminal 31b of the fuel pump relay. The fuel pump relay is energized when power is applied from the START-RUN bus. The relay stays on from ignition pulses applied to terminal 31b. The relay will de-energize one to two seconds after the pulses stop. So you should have power at 31b when the engine is cranking.

You can apply power to terminal 31b and ground terminal 31 and if the fuel pumps run, you know the fuel pump electrical system is good.

If it is not getting power, then you have to look at the ignition system. Maybe a bad Crank Angle Sensor.